This invention relates to process and apparatus for the preparation of transparent electrically conducting thin films for use in a variety of opto-electronic applications such as antistatic coatings, photoconductor storage devices, liquid crystal and electrochromic diaplays, photovoltaic heterojunctions and photothermal absorption devices.
To date, transparent conducting coatings of a variety of oxides such as CdO, SnO.sub.2, In.sub.2 O.sub.3, In.sub.2 O.sub.3 :Sn, and Cd.sub.2 SnO.sub.4, etc., have been prepared by a variety of techniques such as chemical vapor deposition (hydrolysis of chlorides, pyrolysis), evaporation and sputtering of ultra thin metal films (metal films on amorphous substrates, metal films deposited on nucleation modifying layers, post-oxidation of thin metal films), reactive evaporation (evaporation of pure metals, evaporation of oxides), and sputtering (reactive sputtering, sputtering of oxide targets). The status of various transparent conducting coatings can be found in review articles by J.L. Vossen, (Physics of Thin Films, Eds. G. Haas, M. H. Francombe and R. W. Hoffman, Academic Press, Vol. 9, 1, 1977) and G. Haacke, (Ann. Rev. Mater. Sci., 7, 73, 1977).
It is an object of this invention to provide process and apparatus for the production of high quality transparent and electrical conducting coatings such as of In.sub.2 O.sub.3 and Sn:In.sub.2 O.sub.3, by a modified activated reactive evaporation of metal or alloy.
A variety of reactive evaporation processes are reported in the literature for producing a variety of compound films. To the best of applicants knowledge, the only way in which the reactive evaporation technique has been utilized to prepare transparent conducting coatings is simple evaporation in the presence of O.sub.2, (i.e., without the presence of a plasma to activate the reaction). It may be pointed out that the films had poor electrical and optical characteristics (see for example J. L. Vossen, Physics of Thin Films, Eds. G. Haas, M. H. Francombe and R. W. Hoffman, Vol. 9, 1, 1977).
Activation of the reactive evaporation process has been reported in the literature. In U.S. Pat. No. 3,791,852 to Bunshah, thick films of Ti, Hf, V, Nb and Zr carbides are produced by activated reactive evaporation using an electron beam heated evaporation source for metal atoms and hydrocarbon gas for the carbon atoms in the carbide phase. The reaction is activated by placing a positively biased electrode in the reaction zone between the evaporation source and the substrate. The mechanism of activation is as follows: The primary biased electrode (20 to 200 volts) establishes an electric field which extracts low energy secondary electrons from this plasma sheath into the reaction zone. These low energy electrons have a very much higher ionization probability (factor of 10-20) than the high voltage primary electrons, and ionize both the metal and gas atoms to form the corresponding ions. These ions then collide with neutral atoms forming energetic neutral atoms and new ions as a result of charge exchange processes. Thus the reaction zone contains highly active species, i.e., ions and energetic neutrals as well as neutral atoms and electrons. These specie then react to form the compound.
In U.S. Pat. No. 2,920,002 to Auwater, thin film ozides of Si, Zr, Ti, Al, An, and Sn are produced by reactive exaporation. The reaction is activated by passing the reaction gas (O.sub.2) through an ionization chamber located outside the vacuum chamger and having two electrodes with a potential of several thousand volts across them. Magnetic field ionization of O.sub.2 on its way to vacuum chamber has also been used. In U.S. Pat. No. 3,419,487 to Robbins, thin film semiconductors are produced by evaporating a metal and directing a gas into a zone at relatively high pressure (80-90 microns or mercury) and using a high voltage (1470 volts) to generate high energy electrons producing a glow discharge in the zone and subjecting the anode substrate to electron bombardment. Aluninum nitride films have been produced by evaporating Al from an r.f. heater crucible and reacting the Al deposited on the substrate with N.sub.2 gas which has been dissociated by an a.c. discharge on the N.sub.2 feed tube, by M. T. Wank and D. K. Winslow (Appl. Phys. Lett. 13, 286, 1968). Microwave discharge located in the line between the gas source and vacuum chamber has been utilized by B. B. Kosicki and D. Khang (J. Vac. Sci. Tech. 6, 592, 1969, and U.S. Pat. No. 3,551,312) for depositing GaN films.
However none of the prior processes have produced transparent conducting coatings of In.sub.2 O.sub.3 and Sn-doped In.sub.2 O.sub.3 and other similar oxide coatings of low melting point metals by an activated reactive evaporation technique. It is a particular object of the invention to provide process and apparatus which yields films having sheet resistance in the range of 2 to 100 ohms/square and an average transmittance between 0.9 and 0.97.